The building blocks of dyslexia

Psychology professor Iris Berent says we can learn a lot about the reading disorder dyslexia by understanding the intricacies of spoken language. Photo by Mary Knox Merrill.

While dyslexia is most often clas­si­fied as a reading dis­order, it is also well known to affect how indi­vid­uals process spoken lan­guage. Even in infancy, people at high risk for dyslexia seem to have dif­fi­culty pro­cessing speech sounds, according to Iris Berent, pro­fessor of psy­chology in the Col­lege of Sci­ence at Northeastern.

While more than 5 per­cent of the global pop­u­la­tion suf­fers from dyslexia, Berent said much remains unclear about what causes this dis­order because we don’t com­pletely under­stand how the brain decodes printed lan­guage. There are also mys­teries sur­rounding how the brain deals with spoken lan­guage, but lin­guis­tics tells us it involves at least two dif­ferent sys­tems: the pho­netic system and the phono­log­ical system.

“The pho­netic system extracts the dis­tinct building blocks from con­tin­uous acoustic sound,” Berent said. She fur­ther explained that the phono­log­ical system takes those blocks and builds pat­terns with them.

“Think of the metaphor of Lego blocks,” she con­tinued. “The pho­netic system gets the Legos from the plastic stuff; the phono­log­ical system builds pat­terns with them.”

Researchers have long believed that the phono­log­ical system was impaired in people with dyslexia. Yet, sur­pris­ingly, very few studies ever both­ered to check. In a new paper in the journal PLOSONE, Berent and her col­leagues — Vered Vaknin Nus­baum of the Uni­ver­sity of Haifa, Evan Bal­aban of McGill Uni­ver­sity and Albert Gal­aburda of Har­vard Med­ical School — show that the phono­log­ical system of dyslexics is intact. It is actu­ally the pho­netic system that is failing, she said.

To test the phono­log­ical system, Berent and her team pre­sented both skilled and dyslexic readers with the sounds of fake words in Hebrew, the lan­guage used in the study. Of those fake words, some had sound pat­terns that are pos­sible in Hebrew while others are not. The two groups were equally skilled at dis­tin­guishing between them, indi­cating that dyslexic par­tic­i­pants could just as easily iden­tify the Lego pat­terns as those without dyslexia.

Given the existing lit­er­a­ture, this was extremely sur­prising. Indeed, when Berent designed this research, she said she “fully expected to val­i­date the phono­log­ical hypoth­esis.” But the results showed no hint of a phono­log­ical deficit.

Instead, she found a host of subtle prob­lems in the per­cep­tion of speech sounds. For example, in one exper­i­ment, dyslexic indi­vid­uals had a harder time dis­tin­guishing real words from fake words. A second exper­i­ment showed that dyslexics had a hard time dis­tin­guishing real human speech from dig­ital sounds mim­ic­king speech.

“So maybe it’s get­ting the Lego blocks, not pat­terning the Lego blocks, that is impaired,” she said. That is, the problem seems to reside in their pho­netic systems.

To test this hypoth­esis, they also asked lis­teners to dis­tin­guish between dis­crete acoustic sounds (such as “ba” or “pa”). Once again, “there were some reli­able dif­fer­ences in the two groups in iden­ti­fi­ca­tion, and even more so in dis­crim­i­na­tion,” she said.

In the past, dyslexia researchers accepted what Berent calls a “mushy def­i­n­i­tion” of phonology. “Some researchers iden­tify phonology as any process related to speech pro­cessing, whether it is speech per­cep­tion, or the map­ping of let­ters to speech sounds,” she said. “I think the con­tri­bu­tion of our work, is saying, ‘Look at the lin­guis­tics, look at what the two sys­tems really are doing in human lan­guages and maybe that will help you under­stand dyslexia.’”

Berent’s find­ings sug­gest that the dis­order may reside in a lower-​​level com­po­nent of speech per­cep­tion, such as the audi­tory system. Other research sug­gests these dif­fi­cul­ties might arise in the early devel­op­ment of the human brain, per­haps even before birth.

About the Writer

Angela Herring is the science writer for the Northeastern news team. In a past life, she made fullerenes (aka bucky balls) at a small chemical company outside of Boston while freelance writing for the Harvard Stem Cell Institute, the Broad Institute and Novartis Biomedical Research Institutes. She earned her Bachelor's degree in chemistry and literature from Bennington College in 2005. In addition to writing stories for the News@Northeastern, she also maintains the university's research blog: iNSolution.

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